By Louis Charbonneau and Lesley Wroughton
LAUSANNE, Switzerland (Reuters) - Iran and major world powers have been making headway in identifying technical options for a historic nuclear deal as an end-March deadline nears but difficult issues must still be addressed, a senior U.S. official said on Tuesday.
Iran and six world powers are seeking an agreement to curb Iran's most sensitive nuclear activities for at least 10 years in exchange for a gradual end to sanctions on Tehran.
The powers -- Britain, China, France, Germany, Russia and the United States -- aim to complete the framework of a final deal by the end of March and reach a full agreement by June 30.
"We have definitely made progress in terms of identifying technical options for each of the major areas," the U.S. official told reporters on condition of anonymity. "There is no way around it. We still have a ways to go ... But even within this space, we have some tough issues to address."
The official said any framework agreement settled this month would need to have key details, including numbers. "If there is an agreement, I don't see how it could be meaningful without having some quantitative dimensions," he said, without elaborating.
Western and Iranian officials doubted an agreement could be clinched this week and at least one more round of talks would be needed on a deal that could end a 12-year-old nuclear standoff between Tehran and the West over its atomic weapons.
The goal of the negotiations is to arrive at an arrangement whereby Iran would need at least one year to produce enough fissile material -- high enriched uranium or plutonium -- for a single atomic weapon, should Tehran choose to produce one. That is known as the "break-out" time.
The official said the six powers, which have been negotiating with Iran since October 2013, do not share their individual methods of calculating break-out time for Tehran. But they have all reached the same conclusions, he noted.
The official offered no details on the options under discussion. But negotiators say they are looking at a maximum number of enrichment centrifuges Iran could retain, the size of its future uranium stockpiles and other limitations Tehran would be subject to for at least 10 years.
The head of Iran's Atomic Energy Organization, Ali Akbar Salehi, was more upbeat after meetings with U.S. Energy Secretary Earnest Moniz in the Swiss city of Lausanne, where negotiations are taking place.
"We have made progress on technical issues," Salehi told reporters. "One or two issues remain and need to be discussed."
The U.S. and Iranian delegations led by U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif began another round of talks on Tuesday after the Iranians returned overnight from Brussels where they had met European foreign ministers.
Senior European officials were expected in Lausanne later on Tuesday. Officials said foreign ministers are on stand-by to join them at the end of the week if needed but expectations were very low that a deal would come this week.
A senior Iranian official said European foreign ministers would not be joining the talks this week.
"We’ll see what happens the rest of the week but for now we’re not there," a senior Western diplomat said. The U.S. official said the sides would work through the end of the month if needed to secure a deal.
Western powers and their allies suspect Tehran of wanting to create an atomic weapons capability. Tehran denies that and says its nuclear ambitions are purely peaceful.
Israeli Prime Minister Benjamin Netanyahu, who is fighting to remain in power in Tuesday's election in Israel, has made clear he opposes engagement with Iran and enjoys strong support in the U.S. Congress, where Republicans control both houses.
Last week 47 Republicans wrote to Iran's leadership to warn them that they could undo any deal President Barack Obama strikes with Tehran. Another U.S. official said Zarif raised the "ill-timed and ill-advised" letter on Monday with Kerry.
With the Iranian new year holiday of Norouz approaching this weekend, officials close to the talks say it will be difficult to complete a political agreement this week. If it is not possible by the weekend, the talks could reconvene in the final days of March.
Zarif said all sides needed to keep talking this week to see what could be achieved.
"On some issues we are closer to a solution and based on this we can say solutions are within reach. At the same time, we are apart on some issues," he told the Iranian news agency IRNA.
Sticking points include the level of Iran's enrichment activities and how sanctions would be lifted. Iran wants all U.S., EU and United Nations sanctions lifted immediately.
(Additional reporting by Parisa Hafezi and John Irish; Editing by Giles Elgood)